// Protocol Buffers - Google's data interchange format
// Copyright 2008 Google Inc.  All rights reserved.
// https://developers.google.com/protocol-buffers/
//
// Redistribution and use in source and binary forms, with or without
// modification, are permitted provided that the following conditions are
// met:
//
//     * Redistributions of source code must retain the above copyright
// notice, this list of conditions and the following disclaimer.
//     * Redistributions in binary form must reproduce the above
// copyright notice, this list of conditions and the following disclaimer
// in the documentation and/or other materials provided with the
// distribution.
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// contributors may be used to endorse or promote products derived from
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//
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package com.google.protobuf;

import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.SortedMap;
import java.util.TreeMap;

/**
 * A custom map implementation from FieldDescriptor to Object optimized to minimize the number of
 * memory allocations for instances with a small number of mappings. The implementation stores the
 * first {@code k} mappings in an array for a configurable value of {@code k}, allowing direct
 * access to the corresponding {@code Entry}s without the need to create an Iterator. The remaining
 * entries are stored in an overflow map. Iteration over the entries in the map should be done as
 * follows:
 *
 * <pre>{@code
 * for (int i = 0; i < fieldMap.getNumArrayEntries(); i++) {
 *   process(fieldMap.getArrayEntryAt(i));
 * }
 * for (Map.Entry<K, V> entry : fieldMap.getOverflowEntries()) {
 *   process(entry);
 * }
 * }</pre>
 * <p>
 * The resulting iteration is in order of ascending field tag number. The object returned by {@link
 * #entrySet()} adheres to the same contract but is less efficient as it necessarily involves
 * creating an object for iteration.
 *
 * <p>The tradeoff for this memory efficiency is that the worst case running time of the {@code
 * put()} operation is {@code O(k + lg n)}, which happens when entries are added in descending
 * order. {@code k} should be chosen such that it covers enough common cases without adversely
 * affecting larger maps. In practice, the worst case scenario does not happen for extensions
 * because extension fields are serialized and deserialized in order of ascending tag number, but
 * the worst case scenario can happen for DynamicMessages.
 *
 * <p>The running time for all other operations is similar to that of {@code TreeMap}.
 *
 * <p>Instances are not thread-safe until {@link #makeImmutable()} is called, after which any
 * modifying operation will result in an {@link UnsupportedOperationException}.
 *
 * @author darick@google.com Darick Tong
 */
// This class is final for all intents and purposes because the constructor is
// private. However, the FieldDescriptor-specific logic is encapsulated in
// a subclass to aid testability of the core logic.
class SmallSortedMap<K extends Comparable<K>, V> extends AbstractMap<K, V> {

    /**
     * Creates a new instance for mapping FieldDescriptors to their values. The {@link
     * #makeImmutable()} implementation will convert the List values of any repeated fields to
     * unmodifiable lists.
     *
     * @param arraySize The size of the entry array containing the lexicographically smallest
     *                  mappings.
     */
    static <FieldDescriptorType extends FieldSet.FieldDescriptorLite<FieldDescriptorType>>
    SmallSortedMap<FieldDescriptorType, Object> newFieldMap(int arraySize) {
        return new SmallSortedMap<FieldDescriptorType, Object>(arraySize) {
            @Override
            @SuppressWarnings("unchecked")
            public void makeImmutable() {
                if (!isImmutable()) {
                    for (int i = 0; i < getNumArrayEntries(); i++) {
                        final Map.Entry<FieldDescriptorType, Object> entry = getArrayEntryAt(i);
                        if (entry.getKey().isRepeated()) {
                            final List value = (List) entry.getValue();
                            entry.setValue(Collections.unmodifiableList(value));
                        }
                    }
                    for (Map.Entry<FieldDescriptorType, Object> entry : getOverflowEntries()) {
                        if (entry.getKey().isRepeated()) {
                            final List value = (List) entry.getValue();
                            entry.setValue(Collections.unmodifiableList(value));
                        }
                    }
                }
                super.makeImmutable();
            }
        };
    }

    /**
     * Creates a new instance for testing.
     *
     * @param arraySize The size of the entry array containing the lexicographically smallest
     *                  mappings.
     */
    static <K extends Comparable<K>, V> SmallSortedMap<K, V> newInstanceForTest(int arraySize) {
        return new SmallSortedMap<K, V>(arraySize);
    }

    private final int maxArraySize;
    // The "entry array" is actually a List because generic arrays are not
    // allowed. ArrayList also nicely handles the entry shifting on inserts and
    // removes.
    private List<Entry> entryList;
    private Map<K, V> overflowEntries;
    private boolean isImmutable;
    // The EntrySet is a stateless view of the Map. It's initialized the first
    // time it is requested and reused henceforth.
    private volatile EntrySet lazyEntrySet;
    private Map<K, V> overflowEntriesDescending;
    private volatile DescendingEntrySet lazyDescendingEntrySet;

    /**
     * @code arraySize Size of the array in which the lexicographically smallest mappings are stored.
     * (i.e. the {@code k} referred to in the class documentation).
     */
    private SmallSortedMap(int arraySize) {
        this.maxArraySize = arraySize;
        this.entryList = Collections.emptyList();
        this.overflowEntries = Collections.emptyMap();
        this.overflowEntriesDescending = Collections.emptyMap();
    }

    /**
     * Make this map immutable from this point forward.
     */
    public void makeImmutable() {
        if (!isImmutable) {
            // Note: There's no need to wrap the entryList in an unmodifiableList
            // because none of the list's accessors are exposed. The iterator() of
            // overflowEntries, on the other hand, is exposed so it must be made
            // unmodifiable.
            overflowEntries =
                    overflowEntries.isEmpty()
                            ? Collections.<K, V>emptyMap()
                            : Collections.unmodifiableMap(overflowEntries);
            overflowEntriesDescending =
                    overflowEntriesDescending.isEmpty()
                            ? Collections.<K, V>emptyMap()
                            : Collections.unmodifiableMap(overflowEntriesDescending);
            isImmutable = true;
        }
    }

    /**
     * @return Whether {@link #makeImmutable()} has been called.
     */
    public boolean isImmutable() {
        return isImmutable;
    }

    /**
     * @return The number of entries in the entry array.
     */
    public int getNumArrayEntries() {
        return entryList.size();
    }

    /**
     * @return The array entry at the given {@code index}.
     */
    public Map.Entry<K, V> getArrayEntryAt(int index) {
        return entryList.get(index);
    }

    /**
     * @return There number of overflow entries.
     */
    public int getNumOverflowEntries() {
        return overflowEntries.size();
    }

    /**
     * @return An iterable over the overflow entries.
     */
    public Iterable<Map.Entry<K, V>> getOverflowEntries() {
        return overflowEntries.isEmpty()
                ? EmptySet.<Map.Entry<K, V>>iterable()
                : overflowEntries.entrySet();
    }

    Iterable<Map.Entry<K, V>> getOverflowEntriesDescending() {
        return overflowEntriesDescending.isEmpty()
                ? EmptySet.<Map.Entry<K, V>>iterable()
                : overflowEntriesDescending.entrySet();
    }

    @Override
    public int size() {
        return entryList.size() + overflowEntries.size();
    }

    /**
     * The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
     *
     * <p>{@inheritDoc}
     */
    @Override
    public boolean containsKey(Object o) {
        @SuppressWarnings("unchecked") final K key = (K) o;
        return binarySearchInArray(key) >= 0 || overflowEntries.containsKey(key);
    }

    /**
     * The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
     *
     * <p>{@inheritDoc}
     */
    @Override
    public V get(Object o) {
        @SuppressWarnings("unchecked") final K key = (K) o;
        final int index = binarySearchInArray(key);
        if (index >= 0) {
            return entryList.get(index).getValue();
        }
        return overflowEntries.get(key);
    }

    @Override
    public V put(K key, V value) {
        checkMutable();
        final int index = binarySearchInArray(key);
        if (index >= 0) {
            // Replace existing array entry.
            return entryList.get(index).setValue(value);
        }
        ensureEntryArrayMutable();
        final int insertionPoint = -(index + 1);
        if (insertionPoint >= maxArraySize) {
            // Put directly in overflow.
            return getOverflowEntriesMutable().put(key, value);
        }
        // Insert new Entry in array.
        if (entryList.size() == maxArraySize) {
            // Shift the last array entry into overflow.
            final Entry lastEntryInArray = entryList.remove(maxArraySize - 1);
            getOverflowEntriesMutable().put(lastEntryInArray.getKey(), lastEntryInArray.getValue());
        }
        entryList.add(insertionPoint, new Entry(key, value));
        return null;
    }

    @Override
    public void clear() {
        checkMutable();
        if (!entryList.isEmpty()) {
            entryList.clear();
        }
        if (!overflowEntries.isEmpty()) {
            overflowEntries.clear();
        }
    }

    /**
     * The implementation throws a {@code ClassCastException} if o is not an object of type {@code K}.
     *
     * <p>{@inheritDoc}
     */
    @Override
    public V remove(Object o) {
        checkMutable();
        @SuppressWarnings("unchecked") final K key = (K) o;
        final int index = binarySearchInArray(key);
        if (index >= 0) {
            return removeArrayEntryAt(index);
        }
        // overflowEntries might be Collections.unmodifiableMap(), so only
        // call remove() if it is non-empty.
        if (overflowEntries.isEmpty()) {
            return null;
        } else {
            return overflowEntries.remove(key);
        }
    }

    private V removeArrayEntryAt(int index) {
        checkMutable();
        final V removed = entryList.remove(index).getValue();
        if (!overflowEntries.isEmpty()) {
            // Shift the first entry in the overflow to be the last entry in the
            // array.
            final Iterator<Map.Entry<K, V>> iterator = getOverflowEntriesMutable().entrySet().iterator();
            entryList.add(new Entry(iterator.next()));
            iterator.remove();
        }
        return removed;
    }

    /**
     * @param key The key to find in the entry array.
     * @return The returned integer position follows the same semantics as the value returned by
     * {@link java.util.Arrays#binarySearch()}.
     */
    private int binarySearchInArray(K key) {
        int left = 0;
        int right = entryList.size() - 1;

        // Optimization: For the common case in which entries are added in
        // ascending tag order, check the largest element in the array before
        // doing a full binary search.
        if (right >= 0) {
            int cmp = key.compareTo(entryList.get(right).getKey());
            if (cmp > 0) {
                return -(right + 2); // Insert point is after "right".
            } else if (cmp == 0) {
                return right;
            }
        }

        while (left <= right) {
            int mid = (left + right) / 2;
            int cmp = key.compareTo(entryList.get(mid).getKey());
            if (cmp < 0) {
                right = mid - 1;
            } else if (cmp > 0) {
                left = mid + 1;
            } else {
                return mid;
            }
        }
        return -(left + 1);
    }

    /**
     * Similar to the AbstractMap implementation of {@code keySet()} and {@code values()}, the entry
     * set is created the first time this method is called, and returned in response to all subsequent
     * calls.
     *
     * <p>{@inheritDoc}
     */
    @Override
    public Set<Map.Entry<K, V>> entrySet() {
        if (lazyEntrySet == null) {
            lazyEntrySet = new EntrySet();
        }
        return lazyEntrySet;
    }

    Set<Map.Entry<K, V>> descendingEntrySet() {
        if (lazyDescendingEntrySet == null) {
            lazyDescendingEntrySet = new DescendingEntrySet();
        }
        return lazyDescendingEntrySet;
    }

    /**
     * @throws UnsupportedOperationException if {@link #makeImmutable()} has has been called.
     */
    private void checkMutable() {
        if (isImmutable) {
            throw new UnsupportedOperationException();
        }
    }

    /**
     * @return a {@link SortedMap} to which overflow entries mappings can be added or removed.
     * @throws UnsupportedOperationException if {@link #makeImmutable()} has been called.
     */
    @SuppressWarnings("unchecked")
    private SortedMap<K, V> getOverflowEntriesMutable() {
        checkMutable();
        if (overflowEntries.isEmpty() && !(overflowEntries instanceof TreeMap)) {
            overflowEntries = new TreeMap<K, V>();
            overflowEntriesDescending = ((TreeMap<K, V>) overflowEntries).descendingMap();
        }
        return (SortedMap<K, V>) overflowEntries;
    }

    /**
     * Lazily creates the entry list. Any code that adds to the list must first call this method.
     */
    private void ensureEntryArrayMutable() {
        checkMutable();
        if (entryList.isEmpty() && !(entryList instanceof ArrayList)) {
            entryList = new ArrayList<Entry>(maxArraySize);
        }
    }

    /**
     * Entry implementation that implements Comparable in order to support binary search within the
     * entry array. Also checks mutability in {@link #setValue()}.
     */
    private class Entry implements Map.Entry<K, V>, Comparable<Entry> {

        private final K key;
        private V value;

        Entry(Map.Entry<K, V> copy) {
            this(copy.getKey(), copy.getValue());
        }

        Entry(K key, V value) {
            this.key = key;
            this.value = value;
        }

        @Override
        public K getKey() {
            return key;
        }

        @Override
        public V getValue() {
            return value;
        }

        @Override
        public int compareTo(Entry other) {
            return getKey().compareTo(other.getKey());
        }

        @Override
        public V setValue(V newValue) {
            checkMutable();
            final V oldValue = this.value;
            this.value = newValue;
            return oldValue;
        }

        @Override
        public boolean equals(Object o) {
            if (o == this) {
                return true;
            }
            if (!(o instanceof Map.Entry)) {
                return false;
            }
            @SuppressWarnings("unchecked")
            Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
            return equals(key, other.getKey()) && equals(value, other.getValue());
        }

        @Override
        public int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
        }

        @Override
        public String toString() {
            return key + "=" + value;
        }

        /**
         * equals() that handles null values.
         */
        private boolean equals(Object o1, Object o2) {
            return o1 == null ? o2 == null : o1.equals(o2);
        }
    }

    /**
     * Stateless view of the entries in the field map.
     */
    private class EntrySet extends AbstractSet<Map.Entry<K, V>> {

        @Override
        public Iterator<Map.Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        @Override
        public int size() {
            return SmallSortedMap.this.size();
        }

        /**
         * Throws a {@link ClassCastException} if o is not of the expected type.
         *
         * <p>{@inheritDoc}
         */
        @Override
        public boolean contains(Object o) {
            @SuppressWarnings("unchecked") final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
            final V existing = get(entry.getKey());
            final V value = entry.getValue();
            return existing == value || (existing != null && existing.equals(value));
        }

        @Override
        public boolean add(Map.Entry<K, V> entry) {
            if (!contains(entry)) {
                put(entry.getKey(), entry.getValue());
                return true;
            }
            return false;
        }

        /**
         * Throws a {@link ClassCastException} if o is not of the expected type.
         *
         * <p>{@inheritDoc}
         */
        @Override
        public boolean remove(Object o) {
            @SuppressWarnings("unchecked") final Map.Entry<K, V> entry = (Map.Entry<K, V>) o;
            if (contains(entry)) {
                SmallSortedMap.this.remove(entry.getKey());
                return true;
            }
            return false;
        }

        @Override
        public void clear() {
            SmallSortedMap.this.clear();
        }
    }

    private class DescendingEntrySet extends EntrySet {
        @Override
        public Iterator<java.util.Map.Entry<K, V>> iterator() {
            return new DescendingEntryIterator();
        }
    }

    /**
     * Iterator implementation that switches from the entry array to the overflow entries
     * appropriately.
     */
    private class EntryIterator implements Iterator<Map.Entry<K, V>> {

        private int pos = -1;
        private boolean nextCalledBeforeRemove;
        private Iterator<Map.Entry<K, V>> lazyOverflowIterator;

        @Override
        public boolean hasNext() {
            return (pos + 1) < entryList.size()
                    || (!overflowEntries.isEmpty() && getOverflowIterator().hasNext());
        }

        @Override
        public Map.Entry<K, V> next() {
            nextCalledBeforeRemove = true;
            // Always increment pos so that we know whether the last returned value
            // was from the array or from overflow.
            if (++pos < entryList.size()) {
                return entryList.get(pos);
            }
            return getOverflowIterator().next();
        }

        @Override
        public void remove() {
            if (!nextCalledBeforeRemove) {
                throw new IllegalStateException("remove() was called before next()");
            }
            nextCalledBeforeRemove = false;
            checkMutable();

            if (pos < entryList.size()) {
                removeArrayEntryAt(pos--);
            } else {
                getOverflowIterator().remove();
            }
        }

        /**
         * It is important to create the overflow iterator only after the array entries have been
         * iterated over because the overflow entry set changes when the client calls remove() on the
         * array entries, which invalidates any existing iterators.
         */
        private Iterator<Map.Entry<K, V>> getOverflowIterator() {
            if (lazyOverflowIterator == null) {
                lazyOverflowIterator = overflowEntries.entrySet().iterator();
            }
            return lazyOverflowIterator;
        }
    }

    /**
     * Reverse Iterator implementation that switches from the entry array to the overflow entries
     * appropriately.
     */
    private class DescendingEntryIterator implements Iterator<Map.Entry<K, V>> {

        private int pos = entryList.size();
        private Iterator<Map.Entry<K, V>> lazyOverflowIterator;

        @Override
        public boolean hasNext() {
            return (pos > 0 && pos <= entryList.size()) || getOverflowIterator().hasNext();
        }

        @Override
        public Map.Entry<K, V> next() {
            if (getOverflowIterator().hasNext()) {
                return getOverflowIterator().next();
            }
            return entryList.get(--pos);
        }

        @Override
        public void remove() {
            throw new UnsupportedOperationException();
        }

        /**
         * It is important to create the overflow iterator only after the array entries have been
         * iterated over because the overflow entry set changes when the client calls remove() on the
         * array entries, which invalidates any existing iterators.
         */
        private Iterator<Map.Entry<K, V>> getOverflowIterator() {
            if (lazyOverflowIterator == null) {
                lazyOverflowIterator = overflowEntriesDescending.entrySet().iterator();
            }
            return lazyOverflowIterator;
        }
    }

    /**
     * Helper class that holds immutable instances of an Iterable/Iterator that we return when the
     * overflow entries is empty. This eliminates the creation of an Iterator object when there is
     * nothing to iterate over.
     */
    private static class EmptySet {

        private static final Iterator<Object> ITERATOR =
                new Iterator<Object>() {
                    @Override
                    public boolean hasNext() {
                        return false;
                    }

                    @Override
                    public Object next() {
                        throw new NoSuchElementException();
                    }

                    @Override
                    public void remove() {
                        throw new UnsupportedOperationException();
                    }
                };

        private static final Iterable<Object> ITERABLE =
                new Iterable<Object>() {
                    @Override
                    public Iterator<Object> iterator() {
                        return ITERATOR;
                    }
                };

        @SuppressWarnings("unchecked")
        static <T> Iterable<T> iterable() {
            return (Iterable<T>) ITERABLE;
        }
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }

        if (!(o instanceof SmallSortedMap)) {
            return super.equals(o);
        }

        SmallSortedMap<?, ?> other = (SmallSortedMap<?, ?>) o;
        final int size = size();
        if (size != other.size()) {
            return false;
        }

        // Best effort try to avoid allocating an entry set.
        final int numArrayEntries = getNumArrayEntries();
        if (numArrayEntries != other.getNumArrayEntries()) {
            return entrySet().equals(other.entrySet());
        }

        for (int i = 0; i < numArrayEntries; i++) {
            if (!getArrayEntryAt(i).equals(other.getArrayEntryAt(i))) {
                return false;
            }
        }

        if (numArrayEntries != size) {
            return overflowEntries.equals(other.overflowEntries);
        }

        return true;
    }

    @Override
    public int hashCode() {
        int h = 0;
        final int listSize = getNumArrayEntries();
        for (int i = 0; i < listSize; i++) {
            h += entryList.get(i).hashCode();
        }
        // Avoid the iterator allocation if possible.
        if (getNumOverflowEntries() > 0) {
            h += overflowEntries.hashCode();
        }
        return h;
    }
}
